i used a bit thats maybe 1 1/2" long in an extension on a ratchet..a small section ratchet and extension (extension is a little over 1/4" diameter) worked a treat...

I sauspect that at this point getting a GOOD torx bit that is the right size and having a go is a good idea in that you have little to loose....

if that wont shift it i guess your into trying to cut a slot in the head and screwdriver it out, or drill the head off...remove the stat cover then try to remove the stud with pliers (or by threading anut onto it...)

i would defianately replace the bolt once your done..and if it gets to slotting or drilling you obviously need a replacement bolt on hand (and i would suggest a repalcement theromstat cover just in case....

one other throught for you...if it gets REALLY hairy...
drill the head off the bolt...then drill down the centre ofthe bolt with say a 2mm drill and the tap it...then thread a 2mm bolt into the tap..lock in place with a nut and turn it out...but that would be pretty hairy given the diameter of hole and where it is...

if you have a hold on a torx bit..but you feel its a bit iffy...you could always clean the head up (solvents!) then epoxy a torx bit in place...allow to set properly (say overnight!) then try to unscrew it...the glue might imporve things enough to do it!